Ink jet printing has become recognized as a prominent contender in the digitally controlled, electronic printing arena because of its non-impact, low-noise characteristics, its use of plain paper, and its avoidance of toner transfer and fixing. Other applications, requiring very precise, non-contact liquid pattern deposition, may be served by drop emitters having similar characteristics to very high resolution ink jet printheads. By very high resolution liquid layer patterns, it is meant, herein, patterns formed of pattern cells (pixels) having spatial densities of at least 300 per inch in two dimensions.
Ink jet printing mechanisms can be categorized by technology as either drop-on-demand ink jet or continuous ink jet. The first technology, “drop-on-demand” ink jet printing, provides ink droplets that impact upon a recording surface by using a pressurization actuator (thermal, piezoelectric, etc.). Many commonly practiced drop-on-demand technologies use thermal actuation to eject ink droplets from a nozzle. A heater, located at or near the nozzle, heats the ink sufficiently to boil, forming a vapor bubble that creates enough internal pressure to eject an ink droplet. Other well known drop-on-demand droplet ejection mechanisms include piezoelectric actuators.
Drop-on-demand drop emitter systems are limited in the drop repetition frequency that is sustainable from an individual nozzle. In order to produce consistent drop volumes and to counteract front face flooding, the ink supply is typically held at a slightly negative pressure. The time required to re-fill the drop generation chambers and passages, including some settling time, limits the drop repetition frequency. Drop repetition frequencies ranging up to ˜50 kHz may be possible for drops having volumes of 10 picoLiters (pL) or less. However, a drop frequency maximum of 50 KHz limits the usefulness of drop-on-demand emitters for high quality patterned layer deposition to process speeds below ˜0.5 msec.
The second ink jet technology, commonly referred to as “continuous” ink jet (CIJ) printing, uses a pressurized ink source that produces a continuous stream of ink droplets from a nozzle. The stream is perturbed in some fashion causing it to break up into uniformly sized drops at a nominally constant distance, the break-off length, from the nozzle. Since the source of pressure is remote from the nozzle (typically a pump is used to feed pressurized ink to the printhead), the space occupied by the nozzles is very small. CIJ drop generators do not have a “refill” limitation since the drop formation process occurs after ejection from the nozzle, and thus can operate at frequencies approaching a megahertz.
CIJ drop generators rely on the physics of an unconstrained fluid jet, first analyzed in two dimensions by F. R. S. (Lord) Rayleigh, “Instability of jets,” Proc. London Math. Soc. 10 (4), published in 1878. Lord Rayleigh's analysis showed that liquid under pressure, P, will stream out of a hole, the nozzle, forming a jet of diameter, Dj, moving at a velocity, vj. The jet diameter, Dj, is approximately equal to the effective nozzle diameter, Dn, and the jet velocity is proportional to the square root of the reservoir pressure, P. Rayleigh's analysis showed that the jet will naturally break up into drops of varying sizes based on surface waves that have wavelengths, λ, longer than πDj, i.e. λ≧πDj. Rayleigh's analysis also showed that particular surface wavelengths would become dominate if initiated at a large enough magnitude, thereby “stimulating” the jet to produce mono-sized drops. Individual CU drop generators or low density arrays of CIJ drop generators may be configured to produce the 100's of 1000's of small (<50 pL) drops per second per nozzle, which is one of the requirements needed for high quality patterned layered deposition process speeds above 0.5 msec.
Thermally stimulated CU devices may be fabricated using emerging microelectromechanical (MEMS) fabrication methods and materials. By applying microelectronic fabrication process accuracies to the construction of a thermally stimulated CIJ drop emitter, a liquid pattern deposition apparatus may be provided having a wide range of resolution and process speed capabilities. The physical parameters relating to continuous stream drop formation are constrained within certain boundaries to ensure the capability of providing a desired combination of pattern resolution, grey scale, drop volume uniformity, minimization of mist and spatter, and process speed. Such an apparatus has application for very high speed, photographic quality printing as well as for manufacturing applications requiring the non-contact deposition of high precision patterned liquid layers.
Ink jet printing systems that are capable of printing at different resolutions are known in the market. Such printing systems allow the user to select whether to print in a high quality mode at one print resolution at a certain print speed or in a lower quality mode at a lower print resolution at a higher print speed. Typically the lower quality mode, sometimes referred to as a draft mode, increases the spacing between pixels while printing with the same size drops. As a result, the print quality is reduced not only by the resolution reduction, but also by the lower ink coverage. There are some drop-on-demand (DOD) printing systems in which larger ink drops are used for the printing at the lower resolution to produce similar ink coverage levels in both the high and low quality print modes. A need exists to have a continuous ink jet system capable of printing quality prints at multiple resolutions. A system capable of printing at multiple resolutions needs to have a method for adjusting the spot size on paper to achieve the correct ink laydown and coverage for each of the resolutions.
There are documented systems which print at multiple resolutions using DOD technologies. In one example, as described in European Patent No. 0692386 (Onishi et al.) print using a fixed print droplet volume from a DOD ink jet device. In order to achieve multiple resolutions, ink media pairs are chosen such that the repellency of the ink on the media controls the diameters of the ink dots to the proper size. With this approach to multiple resolution printing, there is no flexibility for ink media selection, and it would be difficult to make quality prints at multiple resolutions on a single media type. U.S. Pat. No. 6,419,336 (Takahashi) describes another system capable of printing at multiple resolutions. In this second example, the volume of print drops formed is varied using a peizo system DOD, and is independently controlled. However, as a DOD technology it is fundamentally limited in the frequency at which drops can be made, thereby limiting the attainable process speeds.
In commonly-assigned U.S. Pat. No. 7,249,829 (Hawkins et al.) describes a drop deposition apparatus capable of forming drops of predetermined volumes having a unit volume, V0, and drops having volumes that are integer multiples of the unit volume mV0 using a continuous ink jet system. The disclosure is related to gray level printing, and does not address the problems associated with using drops of mV0 increments in a multiple resolution continuous printing system.
The need exists for a continuous ink jet system capable of printing high quality images at multiple resolutions at fast process speeds.